1. Field of the Invention
The invention relates to a Viterbi decoding apparatus and a Viterbi decoding method which is used for a maximum likelihood decoding method of a convolution code that is used for, for example, a satellite broadcasting or the like.
2. Description of the Related Art
As one of methods of decoding the convolution code, a Viterbi decoding method is known. The Viterbi decoding method is a maximum likelihood decoding method for the convolution code, whereby an error correction is executed by selecting a series (hereinafter, such a series is referred to as a maximum likelihood path) which is the nearest to a received code series from code series which can be formed from an encoder on a transmission side. That is, a transition diagram (hereinbelow, referred to as a trellis) which is formed on the basis of an encoding method by the encoder on the transmission side is used as a prerequisite. For example, a transition in which a hamming distance from the received code series is the shortest is selected from transitions as a maximum likelihood path which can occur on the transition diagram.
A Viterbi decoding apparatus for executing the Viterbi decoding method comprises: a branch metric calculating circuit for calculating a branch metric, namely, a hamming distance between a path which reaches each state on the trellis and the received code series in response to a clock; an ACS circuit for calculating a state metric on the basis of the branch metric, comparing values of the state metrics, and selecting a maximum likelihood path; a normalizing circuit for normalizing the value of the state metric; a state metric storing circuit for storing the value of the state metric; and a path memory circuit for forming decoding data in accordance with a selection result by the ACS.
In this instance, as a path memory circuit, there are two kinds of circuits of a circuit for executing a register transition method of transferring path selection contents by using a register train and a circuit for executing a method of storing the path selection contents by using an RAM and tracing the storage contents, thereby decoding. Those two kinds of methods will now be described hereinbelow.
In the register transition method which has been used in the general Viterbi decoding apparatus, memory cells each comprising a selector and a register are arranged on the trellis in the path memory circuit and the contents in the register are transferred on the basis of path selection information which is outputted from the ACS circuit. By selecting an output of a maximum likelihood state among outputs of memory cells at the final stage, information for the maximum likelihood path is selected and decoding data is outputted.
Although such a register transition method has an advantage such that a high speed operation can be executed, there is a drawback such that when a cut length increases, a circuit scale becomes very large. Particularly, since an application such that the cut length exceeds 100 has recently been presented, the increase in circuit scale becomes a serious problem.
In recent years, a method of decoding by storing path information by using an RAM (Random Access Memory) and tracing the stored information has been being actively studied. Such a method will now be called a trace-back method hereinlater.
As a method of terminating the encoding of the convolution code, a termination has been known. When the terminated convolution code is Viterbi decoded, the maximum likelihood path can be obtained by decoding by tracing back a path from a state of all 0. In the Viterbi decoding apparatus which has been used so far, when the terminated convolution code is successively received, the decoding is executed by the normal operation in many cases.
In such a decoding method, there is a problem such that when the next convolution code is continuously inputted subsequently to the terminated convolution code, a strict maximum likelihood decoding cannot be executed, so that there is a fear that a decoding error occurs. Since this problem is caused by a point that a termination interval is not considered when the Viterbi decoding is performed, the problem also occurs in both of the above-mentioned register transition method and the trace-back method.
The invention is proposed in consideration of such a circumstance. It is, therefore, an object of the invention to provide a Viterbi decoding apparatus and a Viterbi decoding method which can execute a strict maximum likelihood decoding even for a terminated convolution code.
Methods and systems consistent with the present invention provide a Viterbi decoding apparatus for continuously receiving a terminated convolution code, comprising terminate state path decoding means for certainly decoding a path which reaches a state to be terminated.
Methods and systems consistent with the present invention also provide a Viterbi decoding method for continuously receiving a terminated convolution code, comprising terminate state path decoding step of certainly decoding a path which reaches a state to be terminated.
According to the invention as mentioned above, since the path which reaches the state to be terminated can be certainly decoded, it is possible to execute a maximum likelihood decoding for the terminated convolution code.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.